Lighting installations at an airport or the like emit light signals for the orientation and guidance of aircraft which are on the approach to the airport or are moving on its take-off or landing runways, or taxiways. Lighting installations comprise all the lighting aids which are intended to guarantee safe flight operations and safe taxiing of aircraft in the area of an airport, even when it is dark and/or in poor visibility conditions. In this case a distinction is drawn, inter alia, between approach lighting, glide-angle lighting, threshold lighting, side and center lighting, take-off and landing runway lighting, taxiway lighting, identification lights, hazard lights, obstruction lights and rotating lights.
In order to prevent an aircraft pilot from being irritated by light signals with a fluctuating brightness, a constant current must be supplied to the series circuit of a lighting installation. Constant-current regulators are used for this purpose, which provide a constant rated output current in varying conditions, for example fluctuations in the mains input voltage or frequency, the ambient temperature, the height above sea level of their location, relative air humidity and the applied load.
The German product specification “Konstantstromregler Mikroprozessor-gesteuert: A.06.350d” [Constant-current regulator, microprocessor-controlled], Order No. E10001-T95-A52-V2, issued in 1995 by Siemens A G, discloses a constant-current regulator for supplying series circuits in airport lighting installations with various brightness levels. The constant-current regulator has a power module with thyristors connected back-to-back in parallel, a high-voltage section with an output transformer, and a control module which controls the feed voltage for the output transformer via the thyristors. The control module for this purpose determines a thyristor trigger angle, by means of which the output current is matched to a rated value whose magnitude in turn depends on the selected brightness level.
If lamps having a filament are used as lighting appliances in the lighting installation, for example tungsten-halogen lamps, then there is a certain time interval between the instant at which the constant current is applied and the time at which the filament reaches the full light power, whose duration depends on the selected brightness level of the lamps. This reaction delay occurs, for example, on switching on and in the event of short interruptions in the power supply, where a changeover takes place from mains operation to a standby power supply. Until the full light intensity is reached, however, a pilot is not provided with a usable light signal.
On the one hand, the ICAO (International Civil Aviation Organisation) Standard, Annex 14, Volume I, Paragraph 8.1.4 requires that the light output for certain lighting appliances must be up to 50% reproduced within one second after a short current interruption. Lighting appliances which are supplied from a constant-current regulator cannot satisfy this requirement in the event of current interruptions of more than 270 ms, as a result of the physical characteristics of halogen lamps. On the other hand, the currently existing standards from the FFA (Federal Aviation Association), AC 150/5345-10E, from the IEC (International Electrotechnical Commission), 61822 and from CENELEC (Comite Europeen de Normalisation Electrotechnique), ENV 50231 for constant-current regulators demand that the rated current in the series circuit corresponding to the selected brightness must not be exceeded.